Feedback enhanced plasma spray tool

ABSTRACT

An improved automatic feedback control scheme enhances plasma spraying of powdered material through reduction of process variability and providing better ability to engineer coating structure. The present inventors discovered that controlling centroid position of the spatial distribution along with other output parameters, such as particle temperature, particle velocity, and molten mass flux rate, vastly increases control over the sprayed coating structure, including vertical and horizontal cracks, voids, and porosity. It also allows improved control over graded layers or compositionally varying layers of material, reduces variations, including variation in coating thickness, and allows increasing deposition rate. Various measurement and system control schemes are provided.

RELATED APPLICATIONS

[0001] This application claims priority of provisional application No.60/376,135 filed Apr. 29, 2002, incorporated herein by reference.

[0002] This invention was made with Government support under contractnumber DMI-9713957 awarded by the National Science Foundation. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

[0003] This invention generally relates to a device for spraying amaterial. More particularly, it relates to a device for plasma sprayinga powdered material. Even more particularly, it relates to a device thatincludes sensors and feedback for enhanced plasma spraying of thepowdered material.

BACKGROUND OF THE INVENTION

[0004] Plasma spray is used in manufacturing operations in aerospace,energy, engine, electronic, and biomedical because it can economicallyproduce engineered coatings that protect against wear, reduce friction,and reduce corrosion. The plasma spray coatings are also particularlysuited to protect underlying metals against high temperatureenvironments, such as in jet engines. Either ceramic or metalliccoatings can be formed with plasma spray. However, up until now partscoated with plasma spray have varied substantially from each other. Theindustry has therefore not been able to reliably design parts withtightly specified engineered coating structures or to reproduceablyprovide a desired porosity, crack density, and grain structure. Nor hasthe industry been able to consistently provide sufficient control oversprayed layers so different materials could be provided with tightlyspecified properties and thicknesses or so layers with varyingcomposition were deposited. Looser specifications than desired have beenneeded in production or a substantial fraction of coatings have had tobe reworked. And no tool has been available that provides improvedcontrol over these coating parameters while providing a high depositionrate to reduce cost of sprayed layers.

[0005] One scheme to improve plasma spray process was disclosed in anarticle, “Feedback Control of the Subsonic Plasma Spray Process:Controller Performance,” Fincke, J. R., et. al., Proceedings of the8^(th) National Thermal Spray Conference, September 1995 Houston, pp117-122, in which the author demonstrated the ability to independentlycontrol both the particle velocity and the temperature of the particlescoming from a plasma spray torch. In this article, particle temperaturewas measured and the measurement fed back to adjust torch current. Theparticle velocity measurement was fed back to adjust torch gas flowrate.

[0006] Another scheme to improve plasma spray process was disclosed inan article, “Intelligent Processing of Materials for Thermal BarrierCoatings,” by Y. C. Lau, et al, TBC Workshop 1997, sponsored by the TBCInteragency coordination Committee, NASA Lewis Research Center, in whichthe authors provided an interaction matrix and coordinated manipulationof torch current and plasma gas flow inputs to control both temperatureand velocity.

[0007] While both approaches improved control over plasma spraycoatings, wide variation from part to part and even during the processof spraying a single part remained. Thus, a better system forcontrolling plasma spray is needed, and this solution is provided by thefollowing invention.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to moretightly control coating parameters, such as thickness, porosity, crackdensity, and grain structure;

[0009] It is a further object of the present invention to provide fordetecting and controlling spray pattern shape or spatial distribution ofparticles;

[0010] It is a further object of the present invention to provide asystem that includes feedback of spatial distribution of particles or ofa spatial parameter characteristic of the spatial distribution ofparticles;

[0011] It is a further object of the present invention to provide forfeedback of temperature along with feedback of spatial distribution ofparticles or a spatial parameter characteristic of the spatialdistribution of particles;

[0012] It is a further object of the present invention to provide fordetecting and controlling such state attributes of particles as theirtemperature, velocity, and size along with spatial distribution ofparticles;

[0013] It is a further object of the present invention to provide fordetecting and controlling flux rate of molten particles along withspatial distribution of particles;

[0014] It is a further object of the present invention to increasedeposition rate while controlling the other variables;

[0015] It is a further object of the present invention to provide forcontrol of multiple cross coupled process variables;

[0016] It is a feature of the present invention that process variablesare adjusted based on data from a high density region of the particlespatial distribution or from a centroid of the distribution;

[0017] It is a further feature of the present invention to measurevariables including spray pattern spatial distribution, temperature, orvelocity and to adjust current, plasma gas flow rate, or carrier gasflow rate to control the spray pattern spatial distribution,temperature, or velocity;

[0018] It is a further feature of the present invention that themeasured particle states ate averages or mass weighted averages takenacross the spatial distribution of particles;

[0019] It is an advantage of the present invention that processvariations caused by torch wear or aging can be detected and compensatedby taking measurements and making adjustments throughout a depositionrun;

[0020] It is a further advantage of the present invention that coatingstructure can be engineered to control density, horizontal cracks,vertical cracks, voids, and porosity at different depths of the coating;

[0021] It is a further advantage of the present invention that coatingthickness can be more tightly controlled; and

[0022] It is a further advantage of the present invention that coatingdeposition rate can be increased while maintaining control over othervariables and while maintaining desired coating characteristics.

[0023] It is a further advantage of the present invention that itenables grading of coating composition as a function of coatingthickness

[0024] It is a further advantage of the present invention that itenables optimizing deposition efficiency

[0025] It is a further advantage of the present invention that itenables setting a desired deposition rate and deposition efficiency.

[0026] It is a further advantage of the present invention thatdeposition rate can be maintained by adjusting the feedstock rate fromfeed sources including powders, liquids, suspensions of powders inliquids, or wire.

[0027] These and other objects, features, and advantages of theinvention are accomplished by a method of depositing a material on asubstrate. The method includes the steps of:

[0028] a) providing a plasma spray torch having electrodes;

[0029] b) providing a first gas into said plasma spray torch, said firstgas having a first gas flow rate;

[0030] c) providing a controllable power supply for providing a currentacross said electrodes for generating a plasma in said first flow ofgas;

[0031] d) providing particles of a material;

[0032] e) providing a second gas for carrying said particles anddirecting said second gas carrying said particles into said plasma;

[0033] f) heating said particles in said plasma and accelerating saidparticles from said spray torch with said first gas;

[0034] g) measuring a temperature of said sprayed particles;

[0035] h) measuring a spatial distribution of said particles ormeasuring a location characteristic of said spatial distribution of saidparticles; and

[0036] i) adjusting current from said controllable power supply andadjusting said first gas flow rate or said second gas flow rate toobtain a desired temperature of said sprayed particles and a desiredspatial distribution or a desired location characteristic of saidspatial distribution.

[0037] Another aspect of the invention is a system including a sensor,an automatic controller, an actuator, and an input variable. The sensoris for measuring a spatial distribution of particles or for detecting aspatial parameter characteristic of the spatial distribution ofparticles. The input variable is one that effects the spatialdistribution of particles. The automatic controller is for receiving thespatial distribution or the spatial parameter data from the sensor anddirecting the actuator. The actuator is for adjusting the input variableas directed by the automatic controller based on the spatial data.

[0038] Another aspect of the invention is a system for depositing amaterial on a substrate, comprising a spray torch having electrodes. Thesystem also includes a first gas for injecting into the spray torch, thefirst gas having a first gas flow rate. It also includes a controllablepower supply for providing a current across the electrodes. It alsoincludes a controllable device for injecting a material into a regionadjacent the electrodes, wherein the material is heated in the regionand particles of the material are accelerated from the spray torch withthe first gas. It also includes a first sensor for measuring atemperature of the sprayed particles. It also includes a second sensorfor measuring a spatial distribution of the sprayed particles ormeasuring a location characteristic of the spatial distribution of thesprayed particles. It also includes a current actuator for adjustingcurrent from the controllable power supply. It also includes a firstactuator for adjusting the first gas flow rate and a second actuator foradjusting the controllable device for injecting of the material. It alsoincludes a controller to receive data from the first sensor and thesecond sensor and to direct operation of the first actuator and of thesecond actuator to obtain a desired temperature and a desired spatialdistribution or a desired location characteristic of the spatialdistribution of the sprayed particles.

[0039] Another aspect of the invention is a method of spraying a coatingon a substrate comprising the steps of:

[0040] a) spraying a material with a spray tool to provide a spatialdistribution of sprayed particles;

[0041] b) measuring said spatial distribution of sprayed particles ormeasuring a parameter of said spatial distribution of sprayed particles;and

[0042] c) providing automatic closed loop control over said spatialdistribution of sprayed particles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The foregoing and other objects, features, and advantages of theinvention will be apparent from the following detailed description ofthe invention, as illustrated in the accompanying drawings, in which:

[0044]FIGS. 1a and 1 b are schematic diagrams showing a plasma spraytorch spraying particles on a substrate and showing the various controlelements used in the present invention;

[0045]FIG. 1c is test data from a plasma spray system not having thecontrol features of the present invention showing wide variation inplasma spray torch output parameters;

[0046]FIG. 1d is additional test data from the experiment of FIG. 1cshowing wide variation in coating thickness resulting from the widevariation in plasma spray torch output parameters;

[0047]FIG. 2 is test data from a plasma spray system showing variationof particle flux with temperature showing how that variation changes fordifferent feedrates, and showing how the molten fraction shifts withfeedrate;

[0048]FIG. 3 is test data from a plasma spray system showing how averageparticle temperature and molten fraction change at different feedrates;

[0049]FIG. 4 is test data from a plasma spray system showing variationof particle flux with feedrate;

[0050]FIG. 5 is a three dimensional schematic diagram showing a plasmaspray torch spraying particles on a substrate and showing the varioussensors, feedback loops, and control elements used in the presentinvention;

[0051]FIG. 6 is a prior art diagram showing how each input variable istied to more than one output variable;

[0052]FIGS. 7a and 7 b, is prior art test data from a plasma spraysystem showing input variables vs. output variables; FIG. 7c isadditional test data from a plasma spray system showing input variablesvs. output variables;

[0053]FIG. 8 is a prior art scaled transfer matrix and plot ofinput/output interactions;

[0054]FIG. 9 is a model-based feedforward control showing predictedoutputs and experimental results;

[0055]FIG. 10 is prior art test data showing variation in output statefor repeated nominal input conditions;

[0056]FIG. 11 is prior art test data showing output parameters for bothsequential and non-sequential experiments;

[0057]FIG. 12a is test data showing light intensity emitted by sprayedparticles as a function of radial location for different torch flowrates;

[0058]FIG. 12b is test data showing how temperature and flux light varywith radial position;

[0059]FIG. 13a is test data showing how temperature and velocity varywith radial position or distance from the torch centerline;

[0060]FIG. 13b is test data showing how particle mass flux and particlediameter vary with distance from the torch centerline;

[0061]FIG. 14a, 14 b, 14 c is test data showing how a sudden change incurrent affects temperature, velocity and centroid position;

[0062]FIG. 15 is a flow chart showing how a controller of the presentinvention is tuned;

[0063]FIGS. 16a, 16 b, 16 c are test data showing how a sudden change intemperature set point affects temperature, centroid position, andvelocity, showing that with the control system of the present inventionthe system achieves the new temperature set point and restores thepreviously set centroid position and velocity;

[0064]FIG. 16c is test data showing how the sudden change in temperatureof FIG. 16a affects current, torch gas flow rate, and carrier gas flowrate;

[0065]FIG. 16d is test data showing how distribution of total verticalcrack length is a function of particle temperature and velocity and howwith the control provided with the present invention a desired cracklength can be dialed in by setting temperature and velocity setpoints;

[0066]FIG. 16e is test data showing how coating thickness is a functionof particle temperature and velocity and how with the control providedwith the present invention a desired coating thickness and depositionrate can be dialed in by setting temperature and velocity setpoints;

[0067]FIG. 17a is a schematic diagram showing a real time control systemfor maintaining plasma spray spatial distribution with a centroid sensorand an average particle temperature sensor, with a decoupled controlstructure;

[0068]FIG. 17b is a schematic diagram similar to FIG. 17a showing a realtime control system for maintaining plasma spray spatial distributionwith a centroid sensor, an average particle temperature sensor, and aparticle velocity sensor with a decoupled control structure;

[0069]FIG. 18a is a schematic diagram showing a real time control systemfor maintaining plasma spray spatial distribution with a centroid sensorand an average particle temperature sensor, with a MIMO controlstructure;

[0070]FIG. 18b is a schematic diagram similar to FIG. 17a showing a realtime control system for maintaining plasma spray spatial distributionwith a centroid sensor, an average particle temperature sensor, and aparticle velocity sensor with a MIMO control structure;

[0071]FIG. 19 is a schematic diagram showing a real time control systemfor maintaining plasma spray molten particle spatial distribution with amolten particle centroid sensor and an average particle temperaturesensor, with a decoupled control structure;

[0072]FIG. 20 is a schematic diagram similar to FIG. 17a showing a realtime control system for maintaining plasma spray molten particle spatialdistribution with a molten particle centroid sensor and an averageparticle temperature sensor with a MIMO control structure;

[0073]FIG. 21a is a schematic diagram showing a real time control systemfor maintaining plasma spray molten particle spatial distribution with amolten particle centroid sensor and a molten particle flux sensor with aMIMO control structure for centroid and a decoupled powder feedcontroller;

[0074]FIG. 21b is similar to FIG. 21a but sensors for temperature andvelocity are also provided and fed back through the MIMO controller;

[0075]FIG. 22a is a similar to FIG. 21a but a single MIMO controller isused;

[0076]FIG. 22b is a similar to FIG. 21b but a single MIMO controller isused;

[0077]FIG. 23a is a similar to FIG. 21a but a standoff distancecontroller replaces the powder feed controller;

[0078]FIG. 23b is a similar to FIG. 21b but a standoff distancecontroller replaces the powder feed controller;

[0079]FIG. 24 is a similar to FIG. 23b ut a single MIMO controller isused;

[0080]FIG. 25 is similar to FIG. 23b but sensors for both molten andunmolten particles are provided and decoupled controllers are used toadjust both standoff distance and feedrate;

[0081]FIG. 26 is a similar to FIG. 25 but a MIMO controller replaces thetwo decoupled controllers;

[0082]FIG. 27 is a similar to FIG. 26 but a single MIMO controller isused;

[0083]FIG. 28 is a block diagram showing an alternate presentation ofthe control system of the present invention;

DETAILED DESCRIPTION OF THE INVENTION

[0084] Important input and output process variables of a plasma sprayprocess are schematically illustrated in FIGS. 1a, 1 b, showing torch40, plasma jet or plume 42, and particles 44 traveling from torch 40 tosurface 46. Particles 44 are introduced into torch 40 as a powderthrough injector tube 48 along side 50 of torch 40. Carrier gas 52carriers particles 44 of the powder through tube 48 and into torch 40.Alternatively, wire, liquid or suspension can be used for introducingfeed material. Particles 44 are accelerated when they enter plasma jet42 that passes through torch 44. Particles 44 are heated as they crossthrough plasma jet 42. Some of the particles are heated enough to melt,to provide that a substantial fraction of particles 44 are moltenparticles 56. Spatial distribution of particles 58 varies with distancefrom torch 40. Spatial distribution of particles 58 is affected by flowrate of carrier gas 52 among other conditions of the plasma jet 42.

[0085] Torch 40 includes several torch input parameters or set pointsthat can be controlled by an operator or by an automatic controlprogram, including torch current I, torch gas flow rate Qt, and carriergas flow rate Qc, as shown in box 60. Particle type 61, particle feedrate 62, and standoff distance D are additional input parameters thatcan be controlled by the operator or control program, as also shown inFIGS. 1a, 1 b. Other input parameters that effect coating propertiesinclude, substrate surface temperature 64, substrate surface curvature65, and spray angle 66 of particles relative to surface curvature 65.

[0086] Particles 44 traveling in spatial distribution 58 have severaloutput parameters that can be measured, including particle temperature,particle velocity, and particle size and the distributions 68 of thoseparticle state parameters. Output parameters also include the flux ofmolten particles and unmolten particles (not shown) striking surface 46,molten fraction 56, particle trajectories (not shown), spatialdistributions of particles 58, and rate of cooling after strikingsurface 46 (not shown). Molten particle flux is the mass of moltenparticles per second per square centimeter striking surface 46.

[0087] The present inventors found that the output parameters can becontrolled by setting the input parameters, though as we will see, therelationship can be complex. For example, rate of cooling is determinedby substrate surface temperature 64, as well as surface curvature 65, asshown in FIG. 1a.

[0088] The plasma deposition process includes several relatedinteractions. First there is the interaction between torch 40, thecontrollable torch inputs, and resulting plasma jet 42. Second there isthe interaction between plasma jet 42 and sprayed particles 44,including their spatial distribution 58, their temperature, and theirvelocity. These first two interactions are considered thetorch-to-particle mapping. Third, there is interaction between theseparticle states and substrate surface 46 that determines thecharacteristics of the deposited coating.

[0089] The contributors to these three relationships are shown moreexplicitly in FIG. 1b which shows the relation between plasma jet 42 andthe resulting spatial distribution of particles 58. This spatialdistribution is measured by the intensity of light emitted from the hotparticles, and a light intensity plot as a function of y distance fromtorch centerline 70 is shown in FIG. 1b. The light intensity emitted isa function of particle flux rate, particle cross sectional area, andtemperature.

[0090] The present inventors recognized and demonstrated that parametersof coatings deposited by the plasma spray process is determined bydeposition output variables including shape and lateral displacement ofspatial distribution of molten particles 58′ as well as by such outputvariables as particle temperature, particle velocity, particle sizedistributions, the mass flux of molten particles, and by conditions suchas quality of feed material, cleanliness of the substrate, and substratetemperature.

[0091] They further provided insight into why standard open loop controlsystems—that seek to maintain constant input variables, such as currentand gas flow—have not been particularly successful in providingreproduceable process results or maintaining a consistent highdeposition rate.

[0092] They recognized that one of the complexities of plasma spray isthe cross coupling between the manipulable inputs, including current,gas flow rates, powder feed rates, and standoff distance, and theprocess outputs, such as particle temperature, particle velocity, fluxrates, spatial distribution, and molten fraction they found that whenthe torch current is changed, not only does the particle temperaturechange, so does the particle velocity, distribution pattern, and moltenfraction, which in turn affect coating quality.

[0093] They discovered that an automatic system to measure and controlspecified deposition output variables, including spatial distribution ofparticles in combination with other output variables, substantiallyimproved control over coating characteristics and deposition rate. Theyfound that input variables, including current, plasma gas flow rate,carrier gas flow rate, and powder feed rate, were each linked to severalof the measurable output variables. While this made such control moredifficult they nevertheless provided ways to automatically vary theinput variables to provide and maintain constant output variables.

[0094] They observed that the input/output relations and the couplingthemselves change as the torch ages. Because of these changing relationsand changing coupling, they found that simply having sensors for outputvariables available to the operator did not provide him or her with theinformation needed to determine the change in input levels needed torestore the desired deposition output conditions. The inventorsrecognized that even a computer assisted sort of scheme would bedifficult to implement because of the changing state of the aging torch.Torch wear arises because of the DC arcing between the anode and cathodeelectrodes, and this wear is inherent to the process.

[0095] Another primary source of variation is observed each time thetorch is turned on and off. The output characteristics of sprayedparticles vary as a result of the torch history, and this memory effectis known as hysteresis.

[0096] A series of measurements taken while spraying turbine vanesillustrates the changes in torch properties with age and hysteresis andthe resulting changes in particle state, spray pattern and coatingthickness. The torch state was measured before each of 17 parts wassprayed. Variation in particle temperature, velocity, and centroidposition from part to part is shown in curves 72, 74, 76 respectively ofFIG. 1c. Over the course of 4 days, the temperature varied by 300 C(+/−7%), the velocity by 15 m/sec (+/−7%), and the centroid position by+/−6 mm (44%). These variations reflect aging of the torch, variationintroduced by replacing worn electrode elements, and the hysteresiseffect from turning the torch on and off. Corresponding variation incoating thickness measured at 49 points on the 17 vanes sequentiallysprayed is shown in FIG. 1d, showing the effects of the widely varyingoutput parameters of the torch.

[0097] As in this experiment, plasma spray deposition is typicallyoperated in an open-loop fashion in that actuator set points, such asfor current and gas flow rates, are developed empirically based on theuser's process knowledge and experiments. Thus, there is no automaticadjustment of input levels to maintain the process and the particlestate to adjust for process variations. Nor is there an easy-to-usemethod to determine the required set-points to achieve a desired set ofcoating properties. Process variations that occur include electrode wearover its 40 to 50 hour life operation and further variation introducedby multiple on-off cycles and maintenance.

[0098] The open-loop, or feedforward control of standard plasmaspraying, uses either empirical (such as set by an operator) or modelderived input signals to drive the plant. The advantage of feedforwardcontrol is that is anticipatory, and does not require an error to occurbefore changing the input. The achievable performance of feedforwardcontrol, however, is limited since there is no compensation for modelerrors, changes in the plant, or other disturbances. Here model errorand disturbances can refer to unmodeled nonlinearities (such ashysteresis and/or arcing at a new point within the electrode), dynamics,and process variation such as electrode degradation.

[0099] The present inventors found that they could measure and moretightly control the output variables, including spatial distribution, bysimultaneously adjusting two or more of the input variables to obtainand maintain control over each of the desired output variables. Theythereby substantially improved control, improved run to run consistencyof the coatings, and successfully adjusted input variables to overcomewear and aging of the torch and hysteresis, torch variability thatdepended on its history of use. They also provided a way to engineercoating parameters to improve their characteristics, includingresistance to high temperature or wear, while providing for moreconsistent coating characteristics, higher deposition rates, and lowercost coatings.

[0100] One advantage of this closed loop control system is that theprocess remains fixed over a long deposition run, for example in coatinga large turbine blade for a power generating system, as well frompart-to-part, despite torch wear. Another advantage of the closed loopcontrol is that input variables are automatically adjusted to compensatefor the hysteresis affect.

[0101] The present inventors discovered that when any of the torchinputs are changed, the location of the centroid of the spray pattern,ycentroid in FIG. 1b, and the shape of the spray pattern changes. Thelocation of the centroid of the spray pattern is the location of thecenter of mass of the sprayed particles. They experimentally showed thatthe distribution of particles or a parameter representing thatdistribution, such as the location of the centroid, was an importantparameter to control. They found that changes in spray pattern shape orcentroid location effect coating thickness. In addition, they have shownthat controlling centroid position improves the ability to controldeposition rate. They further found that control over the location ofthe spray pattern is important to ensure that a sensor consistentlytracks the same portion of the spray pattern. Otherwise they found thatthe measurement could really be introducing variation, since they foundthat particle state varies with location from torch centerline 70.

[0102] The present invention provides ability to engineer coatingstructure, such as crack orientation, density and porosity for thermalbarrier coatings while compensating for variations due to both torchaging and hysteresis. The strategy of the advanced control systemdescribed below is to replace controlling input variables, such ascurrent and gas flows, to set points with controlling output variablesto set points. Controlling the particle state, particle temperature andparticle velocity, which directly affects the solidification dynamics,has a much more direct impact on coating structure than controlling theraw inputs to the torch (ie current and flow rates). The presentinventors provide two related strategies in addition to controlling thesubstrate temperature. The first is to control the average particlestates as well as spray pattern characteristics. The second is to filterthe particle sensor data to control the parameters of the moltenparticles that wind up in the coating.

[0103] The present inventors recognized that a large fraction of thesprayed particles are not melted and do not wind up in the coating. Thisfraction is illustrated in the particle flux v. temperaturedistributions of FIG. 2 showing the flux having a temperature aboveTmelt compared to the flux having a temperature below Tmelt. Theytherefore provided a scheme to distinguish between the flux of moltenparticles which wind up in the coating and the flux of unmoltenparticles that have been shown to have an impact on coating quality.

[0104] These measurements are used in a closed loop system to maintain adesired ratio of molten to unmolten particle flux, which provides abasis for optimizing deposition efficiency. Since powder costs can be asignificant contributor to per part spray cost, this is an importantfactor in reducing the production costs.

[0105] They experimentally found from particle state distributionmeasurements that averaged conditions do not represent the moltenfraction, particularly in terms of the temperature distribution, asshown in FIGS. 3 and 4. The average temperature of molten particles issubstantially higher than the average temperature of all particles, asshown in FIG. 3, and the average temperature of molten particlesdecreases more slowly with increasing feedrate than does the averagetemperature of all particles, as shown by the slopes of curves 104 and106. However, the molten percentage decreases rapidly with feedrate, asshown by curve 108. Similarly, the flux of all particles increases muchmore rapidly with feedrate than the flux of molten particles, as shownin FIG. 4

[0106] Thus they devised a feedback strategy that uses statistics ofparticles having a temperature over a certain temperature threshold,typically the melting temperature. However, sensors are not absolute intemperature, and some particles may be partially molten and contributeto the coating, so preferably, the set point can be varied so as tobetter include the particles that are included in the coating. Inaddition, they included other molten particle variables in their controlscheme, including flux rate of molten particles and molten fraction.They found that such a control scheme provides greater ability toengineer coating structure, including crack density, crack orientation,and porosity.

[0107] In terms of manufacturing, it is also important to be able tomaximize deposition rate while also meeting coating qualityrequirements. Operating at maximum deposition rate is important sincemore parts can be coated with the same equipment and labor, thusminimizing cost. However it is critical that these high rates aresustainable independent of torch aging while maintaining the desiredcoating characteristics.

[0108] The present inventors recognized that deposition rate depends onflux of molten particles that will be incorporated into the coating andspray angle. Control of deposition rate also requires an additionalinput beyond those needed to control the particle state, such as powderfeed rate or standoff distance. They found that their closed loopcontrol system allowed them to increase deposition rate, while achievingthe desired coating characteristics of crack density, orientation,porosity, etc.

[0109] Another advantage of the closed loop control system of thepresent invention is that it provides the ability to manufacture morecomplex coating structures, such as multi-layers, graded composition,and structures having different microstructures.

[0110] The present invention thus substantially improves plasma spray byeconomically depositing coatings in which the coating structure can beengineered to meet the specific requirements for a variety ofapplications. The economics are related in part to the high depositionrate and low cost (both capital and operating) of the process. Anexample of the desirability of engineering the coating structure is thecoating structure requirements for thermal barrier coatings used inaircraft and power turbines. In thermal barrier coatings it is desirableto minimize horizontal cracks near the bond coat in order to minimizecoating delamination. It is also desirable to maximize vertical cracksto provide thermal stress relief. It is also desirable to increasehorizontal cracks in the body of the thermal barrier coating to increasethermal resistance. The challenge in terms of plasma spray control is todetermine how to change the deposition conditions to achieve such acomplex structure, as well as how to achieve this in every part that iscoated within the required performance bounds. The present inventionprovides control over the deposition to accomplish this goal.

[0111] Besides controlling the coating microstructure, it is alsoimportant to be able to control coating thickness. For example, thermalbarrier coatings usually require a maximum and a minimum coatingthickness. Too thick a coating may result in coating cracking andfailure, too thin a coating will not achieve the desired thermalprotection. The challenge is to meet these thickness specs in spite ofthe variations due to torch aging and the complex part geometry offeredby curved receiving surfaces. Those parts with too thick a coating canrequire costly hand rework, such as by sanding, which significantlyincreases the cost per part. One way to characterize the need to reducethe process variations is to characterize the standard deviationrelative to the performance limits. The smaller the process variation isrelative to the performance bounds, the less rework needs to be done.

[0112] Since the present inventors found that torch aging and hysteresiswill introduce variation in the molten particle flux over time as wellas from run-to-run, they recognized that torch input parameters shouldbe adjusted to compensate for these variations. One strategy is monitorflux in real time and adjust input variables to maintain flux rate, thuscompensating for the variation before it effects coating thickness. Thescheme can be modified to also compensate for variations in effectiveflux rate due to complex surface curvature, using knowledge of the partshape to specify how the deposition rate set point should be varied withposition. The result of this molten particle flux and deposition ratecontrol system will be to reduce the variation of coating thickness,thus reducing re-work costs and increasing yield. In addition, it alsoprovides for increasing deposition rate while still meeting coatingthickness and other requirements. The importance of maximizing thedeposition rate is that a greater number of parts can be coated usingthe same capital equipment and labor.

[0113] The real-time control of the present invention improvesperformance since any disturbance that has an impact on the controlobjectives is measured while the process is running, and the adjustableinputs to the system are varied to quickly compensate for the change inthat measurement and bring it back to its set point. Thus, the magnitudeof the variation can be significantly reduced, so the coating is almostinvariant to disturbances acting on the system.

[0114] In current plasma spray production apparatus, only limitedmeasurement instrumentation is typically used, such as to measure torchcurrent and voltage. Of importance is the operator's ability tovisualize and hear the torch spray pattern. However, it is interestingto note that data collected at one spray manufacturing facility hasshown that coating thickness variation is actually increased somewhatwhen operators intervene compared to not having the operator intervene.

[0115] Even if a better set of sensors were implemented, it would stillbe a challenging job for the operator to determine how to change thevarious actuator set points, for example how to change torch current,plasma gas flow rate, and carrier gas flow rate, to correct for thevariation observed by sensors. The present invention goes beyondproviding that improved measurement system and direction-it makes theadjustments automatic and independent of operator intervention.

[0116] A major problem is due to the cross coupling between the currentand gas flow inputs controllable by the operator and the particle statesemerging from the spray tool. For example, the present inventors havefound that when current changes, not only does particle temperaturechange, but also particle velocity and the spray pattern spatialdistribution changes. While it is not inconceivable that some systemcould be devised to aid the operator in adjusting the actuator setpoints to account for this cross coupling, it will likely be complex andpossibly fairly involved and time consuming. In contrast, the presentinvention provides the ability for the torch system to adjust itselfautomatically, as shown schematically in FIG. 5. Torch 40 spraysparticles 44 in distribution 58 at surface 46. Particles 44 are measuredwith inflight particle sensors 76 a to provide volumetric mass-weightedaverage temperature. Particles 44 are also measured with individualparticle sensors 76 b to provide particle temperature, velocity andsize. Spray pattern sensor 76 c provides spray pattern shape andcentroid position. Substrate temperature is measured with opticalpyrometer 76 d. Measurements from these sensors are either directlyprovided to controller 78 or parameters, such as deposition rate, arecalculated in estimator 80 and these calculated values are input tocontroller 78 which uses the measurements to adjust actuators 82 thatcontrol torch input parameters, including current, powder fee rate, gasflows, and substrate temperature, and standoff distance to betterachieve the desired output parameters.

[0117] The closed loop, or feedback control illustrated in FIG. 5 offersthe opportunity to automatically compensate for variations as well todirectly achieve the required spray conditions to achieve a desiredcoating structure. Feedback structures use measurements of the systemoutput to continuously develop a corrective input signal by thecontrollers. Feedback control can allow the system to take correctiveaction in the event of a process variation or disturbance. The feedbacksystem can automatically determine the input settings that achieves thedesired outputs. And if output parameter set points are changed, thesystem can follow these command signals. Thus, feedback control providesthe ability to correct for changes in the system and correct formodeling errors.

[0118] A paper by some of the present inventors, “SystemCharacterization and Plasma-Particle Distribution Analysis forDevelopment of a Closed Loop Control for Plasma Spray,” HTD-Vol. 366-3:Proceedings of the ASME: Heat Transfer Division November 2000, Volume 3pp. 419-426 (“the system characterization paper”) describes a process tosimultaneously control several degrees of freedom with severalinputs—referred to as multiple-input-multiple-output (MIMO) systems.This kind of process can pose special difficulties in achieving desiredperformance because of the cross coupling of variables. For plasmadeposition, important interactions include the coupling between theparticle states (temperature and velocity) to the inputs torch current,I, and torch flow rate Qt, as shown schematically in FIG. 6, from thesystem characterization paper. The cross coupling, for example, refersto the fact that when the current is changed, both the particletemperature and velocity are affected, as shown in FIG. 7, also from thesystem characterization paper. Similarly, when the torch flow rate Qt ischanged, both the temperature and velocity are also affected. For plasmaspray control, the input-output coupling is really much more complexbecause there are up to five cross coupled input and five outputvariables, and one should consider all their cross couplinginteractions.

[0119] As described in the system characterization paper, Miller AI 1075ZrO2-8Y2O3 was sprayed using a Miller SG-100 plasma torch, with a 730electrode combination and a 112 gas ring (no swirl). Nominal operatingconditions were: torch current of 800 amps, plasma gas flow of 46 litersper minute (slm) (20 percent helium and 80 percent argon), carrier gasflow of 4 slm of argon and the rotary powder feeder was set at 4 rpm.

[0120] Steady state characterizations of the torch involved mapping therelation of plasma torch inputs to the particle states within the spraypattern (temperature, velocity and spray pattern location). The torchcurrent was varied over the range from 700 to 900 amps, the plasma gasflow from 37 slm to 55 slm (keeping a 20/80 ratio of He/Ar), the carriergas flow from 3 slm to 5.5 slm, and the powder feed was varied from 2-5rpm.

[0121] Sprayed particle flow field diagnostics were obtained at astandoff distance of 10 cm, and included simultaneous measurement ofparticle size, velocity, and temperature, and spray pattern shape.Particle size and velocity were measured with an Aerometrics phaseDoppler particle anemometer (PDPA). Temperatures of individual particlessensed by the PDPA were measured using a two-color pyrometer, withwavelength bands of 950+/−20 nm and 850+/−20 nm. The PDPA measurementvolume was positioned to measure particle velocity in the spraydirection, with a measurement volume that was approximately 1 mm in thedirection transverse to the spray direction. The PDPA measurement volumecould be moved in the transverse direction to provide particle-stateprofiles across the spray pattern. The steady-state input/output datawas taken by moving the measurement volume to the centroid maximum.

[0122] The Inflight Particle Pyrometer (IPP), also a two-color device,was also used to measure temperature. The IPP averages over severalcentimeters in the transverse direction, providing an ensemble averageparticle temperature that represents a weighted spatial average over thewidth of the spray pattern. The IPP was positioned to measuretemperature at the same axial location as the PDPA (10 cm).

[0123] A line-scan camera was used to measure the profile of the radiantintensity of the spray pattern in the transverse direction. Thismeasurement provided a means for determining the size and location ofthe spray pattern and their variations with changing torch conditions.In particular, the centroid of the spray pattern was defined as thecentroid of the signal from the line scan camera.

[0124] Our steady-state input-output study was conducted by varying eachinput while measuring three outputs. Inputs considered were the totalplasma gas flow rate, torch current, and carrier gas flow rate, whileoutputs were the averaged particle temperature and velocity, and thecentroid position of the particle spatial distribution (a total of 9curves). Two representative relations from the system characterizationpaper are shown in FIG. 7a, 7 b in which the current was sequentiallyincreased from 700 amps to the higher current levels.

[0125] A least squares linear fit to the data of all nine curves of FIG.7c yields the transfer matrix, as shown in FIG. 8 from the systemcharacterization paper, which is scaled by allowable perturbations ofthe inputs and sensitivity values for the outputs. These relationshipsare illustrated in FIG. 8 in a plot of the column vectors of G in theoutput space. Here, each vector I, Qt, −Qc reveals how the outputs areaffected when a single input is varied. These relationships are validfor the change of operating values (+/−100 amps and +/−8 slm,corresponding to changes of +/−13% and +/−10% respectively). This plotsuggests that choosing current and torch gas flow rate can allow one toindependently control particle temperature, and velocity. Since −QC andI have similar impact in terms of direction, they should not be pairedtogether.

[0126] The nearly linear input/output mappings found in FIG. 7a, 7 bfrom the system characterization paper suggested that implementing amodel-based feedforward controller would be reasonable. The desiredoutput conditions shown in FIG. 9 (diamonds) from the systemcharacterization paper were chosen to determine the impact ofindependently controlling particle temperature/velocity on the resultingcoating structure (points 8, 1, and 4 are velocity while points 3, 1,and 2 are temperature). While some of the actual output points (squares)were close, others had significant errors (including sign changesrelative the nominal base case).

[0127] Additional steady-state experiments were conducted to determinethe reason for this failure. Instead of a sequential sweep of a singleinput, we varied the input conditions in a non-sequential manner.Plotting the output results for all the nominal input cases, as shown inFIG. 10 from the system characterization paper, revealed that it is notpossible to depart from and then return to a nominal particle state.

[0128] The non-sequential experimental results were also compared to theoriginal transfer matrix results, shown in FIG. 11 from the systemcharacterization paper. The significant variations indicate that thetorch results are highly dependent on the path taken since thesequential tests result in a highly structured relationship and thenon-sequential are not well correlated. This behavior suggests some sortof hysteresis since the output state is dependent on the path the torchtook to get there.

[0129] We concluded that a feedforward based controller could notachieve control of the process within tight bounds and could not ensurethat the desired spray conditions are achieved every time the torch isturned on and off and/or changed. We therefore turned to a measurementbased feedback control system. The hysteresis implies that real-timefeedback control is required to maintain even a nominal point for anextended deposition period (relative to the life of the electrode), orif it is desired to change the nominal particle state to achieve gradedcoating characteristics.

[0130] A light intensity scan across a cross section of plasma jet 42for different torch gas flow rates Qt is shown in FIG. 12a. For each gasflow rate the light intensity varies with radial location, or distancefrom centerline 70 of plasma jet 42, as expected. In addition, the datashows significant shifting of the light intensity vs. radial location astorch gas flow rate changes. The present inventors found similar changesin peak and centroid locations as other input parameters changed,including torch current and carrier gas flow rate.

[0131] The light intensity emission at each point in the spatialdistribution scanned is a complex convolution of particle temperature atthat point and number density of particles, which is related to the fluxof particles. The relationships between particle temperature and numberflux and radial position are illustrated by curves presented in FIG.12b. It is seen that in plasma spray, the temperature peaks at the torchcenterline, which is centerline 70 of plasma jet 42, as illustrated bycurve T of FIG. 12b, which peaks at around 0 mm radial position. Numberdensity and flux of particles, however, have an off-axis maximum ataround 12 mm radial position in curve F because the particles areinjected into the plasma jet from side 50 of torch 40 in carrier gas 52having carrier gas flow rate Qc, giving them a sideways velocity inaddition to the forward velocity they experience from their accelerationin much larger torch gas flow Qt. Thus, the shifting of the maximum andcentroid of light intensity scans from the centerline in FIG. 12a withtorch gas flow rate is therefore fairly representative of the shiftingof the number density, flux, and spatial distribution of particles fromcenterline 70 of plasma jet 42.

[0132] The present inventors recognized that the shifting of the radiallocation of the peak of the light intensity distribution and the radiallocation of the centroid of the light intensity distribution as torchgas flow rate, torch current, or carrier gas flow rate changed meantthat the spatial distribution of particles 58 was shifting as any ofthese parameters changed.

[0133] The present inventors also recognized from these plots thatcontrol of torch outputs must include control over spatial distributionof particles 58 and that choice of control logic, sensors, filteringalgorithms, and control laws should include devices for detecting thatspatial distribution or a parameter such as centroid position, that wascharacteristic of the distribution.

[0134] In addition, in view of the temperature distribution of FIG. 2showing that only a fraction of the YSZ particles have surfacetemperatures above the melting point, and the variation in temperaturewith feed rate shown in FIGS. 2, 3, and 4, the present inventorsrecognized that control should focus on the spatial distribution ofmolten particles 58-the particles that stick to the surface—rather thanon the entire distribution of particles 58 or the average of that entiredistribution. They recognized that controlling to the averages would notbe indicative of the distributions that actually determine the coating.

[0135] The temperature distributions in FIG. 2 also show an increasingfraction of particles having a temperature below the melting point asfeed rate increases. The decline of average temperature of all particlesas feedrate increases is also evident in curve 104 of FIG. 3, while themore gradual decline of average temperature of molten particles withfeedrate is seen in curve 106. Specifically, the average temperature ofall particles declines by 72° C. while the average temperature of moltenparticles declines by only 52° C. as feed rate increases from 2 to 10rpm. Consistent with this is 28% decline in the percentage of moltenparticles over this same change in feed rate, as shown in curve 108.Examination of the relative flux rates in FIG. 4 shows that increasingthe powder feedrate is likely to result in an increase in depositionrate because the flux of molten particles does increase, but theincrease is much more gradual than the increase in flux of allparticles, suggesting a substantial decline in deposition efficiency.

[0136] Thus, the present inventors recognized that for improved controlover deposition rate and coating thickness, it is important to be ableto directly measure the spatial distribution of the molten fraction orparameters indicative of that distribution, such as peak and centroidposition, and that measurement of averaged quantities for all particleswill be misleading. In terms of developing a better understanding of theprocess-property mapping, the data suggest advantage in obtainingdistinctive characteristics of the molten fraction, which are notsatisfactorily revealed by the ensemble averages.

[0137] An illustration of the typical differences between the molten andtotal particle distributions, and the resulting impact on measuringparticle state and mass fluxes is shown in FIGS. 14a, 14 b. This data isobtained from a typical spray condition by measuring the individualparticle statistics in terms of their spatial distribution relative tothe torch centerline (e.g. y coordinate). Curve 110 plots the total massflux rate and curve 112 the total particle diameter, curve 114 themolten mass flux rate, and curve 116 the molten particle diameter.Molten particle flux and diameter are taken for particles having atemperature above the melting point or above a specified thresholdtemperature. These curves were calculated from the measured particletemperature, velocity, and diameter data taken from the Inflight IPM3000 sensor, which also provides a measure of particle flux rate, thenumber of particles per unit area per second. Molten particle mass fluxcurve 114 and molten particle diameter curve 116 are significantlydifferent in form then total mass flux curve 110 and total particlediameter curve 112 of FIG. 13a.

[0138] Similarly, location of the centroid position for molten particlesymelt,cent is substantially shifted from location of the centroidposition for all particles ycent, as also shown in FIG. 13a. Thus, thepresent inventors confirmed the desireability of scanning the particleplume and calculating the molten mass flux and centroid position of themolten mass flux.

[0139] In addition, curve 120 of temperature and curve 122 of velocityof molten particles in the plume differs substantially from curve 124 oftemperature and curve 128 of velocity for all particles, as shown inFIG. 13b, indicated substantially different temperatures and velocitiesfor the two distributions. The present inventors found that measuringand controlling based on measured parameters of the molten particlesgives substantially better results than measuring and controlling basedon measured parameters of all particles.

[0140] The various embodiments of the system for control in the presentinvention use sensors that can measure output parameters, such ascentroid location and average particle velocity and temperature, Thecentroid corresponds to the radial location of the “center of mass” forthe spray pattern. Some or all of these output characteristics are thencontrolled in real time to set points set by the operator or controlprogram. The control is provided by the system automatically varyinginput parameters, including current and gas flows. Because of the crosscoupling effects described herein above, in which changing one inputaffects all of the outputs, controllers that accommodate the crosscoupling are desirable.

[0141] The nature of the cross coupling of the torch-particle mapping isillustrated in FIG. 7c which shows three output variables, particletemperature, velocity, and centroid position, and how they are affectedby a change of each input variable, current, torch gas flow rate, andcarrier gas flow rate, in a sequential scan. The effect of the crosscoupling is seen in that there is a significant change in output valueas each input is changed. Thus, as one looks at a column of the figures,one sees that all outputs are affected when only one input is changed.The one exception is between current and centroid position where thisdata shows that centroid position does not substantially shift as torchcurrent changes.

[0142] Ideally, a control system independently controls each of theoutputs so the user specifies its value and the control system maintainsthat value. However, the cross coupling illustrated in FIG. 7c limitsthe degree to which one can independently control all outputs. The crosscoupling also limits which input variables can be used together tocontrol each output parameter. The input variables chosen should have asignificant impact on the desired output while having lesser impact onother control loops.

[0143] To begin the analysis of control structure, we utilize theinput-output relations that are experimentally obtained for the systemin FIG. 7c which can be expressed in terms of the input/output matrixshown in FIG. 7d. Each number in the matrix represents the steady statevalue between the output and the corresponding input. A large numberthus represents the fact that the input will have a significant impacton that output.

[0144] There are several factors that should be considered to determinewhether it is at all feasible that the outputs can be independentlycontrolled given the selected inputs, and if so, whether a decoupledcontrol structure will work, or whether a more complicated MIMOcontroller that takes the coupling into account must be used.

[0145] The first factor is to evaluate the condition number of thesystem. The condition number is the ratio of the largest singular valueof the transfer matrix G to the smallest singular value of the matrix(FIG. 7d, line 3). A scaled matrix is used in order to reflect theallowable variations in input values, and the desired tolerances of howtight one seeks to regulate the output values. In FIG. 7d, the inputswere scaled (nondimensionalized) by I=100A, Qt=9.2 slm, Qc=1 slm; andthe outputs were scaled by tolerance requirements: T=50C, V=12 m/s,Ycent=l1 mm,

[0146] It is widely know that systems with condition number above 10cannot easily be controlled. In this case, one must decide which of theoutputs must be eliminated as well as choosing which inputs to achievethe best possible control (reflected by reducing the condition number).Thus, for input/output curves identified in FIG. 7C and represented bythe matrix G in FIG. 7d, the condition number is 3.3, indicating that itis reasonable to control all 3 degrees of freedom.

[0147] The second factor is to determine to what degree 3 independentloops can be used without explicitly compensating for the cross couplinginteractions. The problem that could exist is that the cross couplingwill cause the loops to interact, and in some cases, wind up beingunstable. The technique we use is the relative gain array (RGA) which iscalculated from the system matrix G as shown in FIG. 7d, item 5. Eachcomponent of the array is a factor that indicates the appropriateness ofcoupling each output with a particular input. Appropriate choices ofindependent input-output loops are indicated by relations in the RGAthat are positive and values close to 1. Pairings with values close to0.5 will result in large interactions, which might indicate thedesirability of using either a decoupler strategy or more complex MIMOcontrol algorithm (which explicitly takes the cross coupling intoaccount in the control algorithm). For the plasma torch experiments, wefind that for this operating point, one should pair the centroidposition with carrier gas flow rate, particle velocity with torch gasflow rate, and particle temperature with current, although the latterloop will result in a significant impact on particle velocity.

[0148] If a decoupled control system is desired, that is individualloops for each output, one must then select a control algorithm used ineach loop's controller. There are a variety of control algorithms thatare typically used. A common one that is widely available in eitherstand alone hardware, such as the model T630C from Foxboro, nowInvensys, is a PID controller and one must then specify the controlgains used (3 gains to be selected). Selecting gains can be accomplishedautomatically by the controller by providing a step input, measuring aresponse, and then identifying gain and time constant. Alternatively,the user can conduct system identification experiments explicitly, asshown in FIGS. 14a, 14 b, 14 c. The experimenally derived model of theprocess can be used to determine the control gains. Alternatively,internal model control can be used, in which the control gains aredetermined directly from calculations based on the identified model ofthe process.

[0149] Alternatively a decoupler may be designed to minimize the crossimpacts that might adversely affect performance. Such a decoupler can bedesigned using standard strategies used in chemical engineering whichare implemented in terms of an inverse of the system model, similar toInternal Model Control for a multiple-input-multiple output systemdescribed in section 16.3.3 and 17.7 of the book ““Process Modeling,Simulation, and Control for Chemical Engineers”, W. L. Luyben,McGraw-Hill, 1989. Here the model can either be of the steady stateinput-output gains, or it can include dynamic elements that representthe dominant dynamics of the system.

[0150] The simplest form of control structure that accommodates thecoupling uses independent controllers 110 a, 110 b, where each outputloop 112 a, 112 b, taking data from sensors 114 a, 114 b, is tied to asingle input summing junction 116 a, 116 b, as shown in FIG. 17a.Preferably, one should characterize all the input/output relations todetermine which input is most strongly tied to which output, as shown inFIG. 8. This knowledge is then used to select the appropriateinput-output pairings, as illustrated in FIG. 17a where current is usedto control temperature and torch gas flow Qt or carrier gas flow Qc isused to control centroid position ycent. Temperature sensor 114 a readsaverage particle temperature.

[0151] Temperature sensor 114 a is can be a 2 color pyrometer thatimages a large volume of light emitted from the particles to get aspatial average of those particles in an instant in time. Such anInflight Particle Pyrometer (IPP) sensor is available from Inflight Ltd.Additionally, any of several sensors that measure individual particles,such as the Inflight IPM-3000 can use a temporal average of theindividual data to provide an average temperature. However, some sensorslimit how fast the controller may operate. The average particletemperature reading is subtracted from desired and preset averageparticle temperature setpoint 118 a in summing junction 116 a.

[0152] Summing junction 116 a is implemented either as an analogcircuit, such as by an op-amp, or as a digital algorithm implemented ona computer or DSP chip. The difference or error in average temperaturefrom summing junction 116 a is fed to automatic controller 110 a whichhas an algorithm to change power supply current I to torch 40. Automaticcontroller 110 a is a PID controller. A typical power supply that isused in plasma spray is the Praxair PS-1000 power supply which iscapable of delivering up to 1500 amps of electrical current. The actualoutput is determined by a 0-10 volt signal supplied by an externalsource, here our control system. The power supply maintains thespecified current to within 1 percent of the set-point determined by the0-10 volt signal.

[0153] The hardware needed to implement the different controlarchitectures require the ability to take the various sensor signals(primarily as analog signals, but also possible as digital words) in,calculate the different error values (i.e. summing the set point to thenegative of the measured signal), and then using this error signal in acontrol algorithm (such as a standard PID control equation or a morecomplex MIMO based control algorithm).

[0154] Stand-alone PID controllers, as well as a more integrated dataacquisition and control system, such as offered by National Instruments,can be used.

[0155] The control system reads sensor signals, including particle state(temperature, velocity, and diameter), centroid position, molten fluxrate, and particle flux rate. Some of these signals, such as centroidposition, mass averaged values of temperature and velocity, and moltenflux, might actually be computed as part of the data acquisition andcontrol system algorithms instead of directly by the sensor. Inaddition, it is typical practice to also record the corresponding valuesof torch primary gas flow rate, torch secondary gas flow rate carriergas flow rate, torch current, torch voltage, substrate temperature, aswell as other system operating conditions, which can be accomplished bythe same control hardware. One or more gases may be used for forming theplasma, including nitrogen, argon, and hydrogen. The gases may be usedin combination. To implement the control algorithm the controllercommunicates with to the various actuators, including those controllingtorch primary gas flow rate, torch secondary gas flow rate, carrier gasflow rate, torch current, and powder feed rate. These signals are alsotypically analog 0-10 volt signals. Digital words can also be used, suchas for the powder feeder.

[0156] One system that is capable of the required input/ouput andcomputational tasks to implement the control architectures of thisinvention is the National Instruments (NI) data acquisition and controlcards. These cards enable a host computer to run the NI Labviewsoftware, which implements either standard control algorithms (such asPID) or enables the user to specify their own code, such as required forimplementing model based Internal Model Control or decoupler strategies.For example, one could use the DaqCard6715 Analog Output Card whichdelivers +/−lOVolts to 8 channels with 12 bits of resolution. Outputvalues can be updated at a rate of 1 Mhz. The various sensors can beconnected to a PCI-MIO-16E-4 and DaqCard-AI-16E-4 Multifunction DataAcquisition Cards. Both devices are capable of measuring +/−10 Voltsignals with 12 bits of resolution as well as digital inputs. Signalsare measured at a rate up to 250 kHz.

[0157] An alternative to implementing the control algorithm on astandard personal computer and on National Instruments input/outputcards is to use a special card with its own microprocessor to implementand solve the control algorithms independently of the host computer.National Instruments' PCI-7030E is capable of monitoring 16+/−10Vsignals and controlling 2+/−10V signals with 16 bit resolution andsampling/updating rates at 100 kHz. The device contains 8 Mb of memoryfor control algorithms which are downloaded from the host computer.

[0158] It is also possible to implement the control algorithms usingindependent process controllers which operate as stand-alone devices.Red Lion Controls manufactures a line of modular process controldevices. The CSMSTR module is capable of hosting up to 16 CSPID modules.Each CSPID module accepts one 0-10V process signal with 16 bitresolution and outputs one 0-10V command signal with 500 μV resolution.Both input and outputs sample/update at 15 Hz. PID control gains can bepredetermined and set on the device via Ethernet communications and apersonal computer.

[0159] Spatial distribution sensor 114 b detects position of centroid ofspatial distribution 58. Spatial distribution sensor 114 b can bemeasured with a line scan CCD camera, such as that supplied by Inflightin their Torch Diagnostic System (TDS), that measures the profile of theradiant intensity of the spray pattern in the traverse direction. Thispattern is used to determine the spray pattern centroid, maximum (e.g.peak) and half width that characterize the spray pattern. The centroidmay also be calculated from individual particle measurements obtained bya sensor that traverses the spray pattern, by explicitly calculating thedistribution obtained by individual particles as a function of position.In this case, one must set the distance between positions and take datafor a fixed amount of time. This method also provides the option ofcalculating the centroid position for particles above a specifictemperature such as the melting point, as shown in FIG. 13a to have adifferent distribution and position then for of that for all theparticles. Individual particle data maybe taken by instruments fromseveral vendors such as the DPV-2000 from Tecnar and the InflightParticle Monitor (IPM-3000). These sensors measure particle temperature,velocity, and diameter which may then be averaged and used to determineboth the spatial averages used for control and the centroid position.Sensors maybe based on single point measurements wherein the sensor headis scanned to image an entire plume.

[0160] The centroid position reading is subtracted from desired andpreset centroid position setpoint 118 b in summing junction 116 bimplemented either as an analog circuit such as by an op-amp, or as adigital algorithm implemented on a computer or DSP chip. The differenceor error in centroid position from summing junction 116 a is fed toautomatic controller 110 b, also a PID controller which has an algorithmto adjust torch gas flow or carrier gas flow to torch 40. Gas flows maybe regulated based on a 0-10 volt signal from a control system utilizingstandard mass flow controllers such as those from MKS or UnitInstruments. These devices are calibrated to work for specific gasesused in the torch, as well as over a specific gas flow range. They takeinputs from the central gas mass flow control/power supply such as UnitInstruments URS-100-5 Readout Power Supply. Each mass flow controlaccepts a 0-5V command signal and outputs a 0-5V actual flow signal,interpreted as a fraction of the mass flow controller's maximumcalibrated flow rate. The URS-100-5 supplies not only the power for theindividual MFC's, but also implements the local control algorithm thatdetermines how each mass flow controller should respond to both thecommand signal and the measured response. The URS-100-5 operates in anautomatic mode, where it supplies the 0-5V command signal to the massflow controllers in accordance to an external computer-supplied setpoint signal.

[0161] A separate control algorithm is implemented for each controlloops. Control gains used in that algorithm are experimentallydetermined. An off the shelf PID controller, such as the Foxboro T630Cmay be used for contoller. The three controller gains, for theproportional, integral, and derivative actions of the controller aredetermined experimentally, as illustrated in the flow chart in FIG. 15,which shows the steps used to determine these three PID control gainsfor the various embodiments of control described in FIGS. 17a-27.

[0162] As shown in step 90 of FIG. 15 the first step is toexperimentally determine system response characteristics. This includesmeasuring the input/output relationships, as illustrated in FIG. 7c,obtaining gains, from the slopes of the curves. Similarly, it includesmeasuring timing, as illustrated in FIGS. 14a-14 c, and determining timeconstants for all input/output relationships.

[0163] In the next step, performance objectives to meet plasma sprayrequirements are determined, as shown in step 91. This step isaccomplished by specifying desired system settling time, permittedsteady state error, how far a parameter can overshoot the set point whenfollowing a command, and robustness to model error. These parameters areset to allow control for spray coating a part to take a reasonableamount of time.

[0164] In the next step, a trade-off analysis is performed, as shown instep 92, that uses the performance objectives of step 91 and the systemresponse characterists experimentally determined in step 90 to determineachievable closed loop objectives for the PID controller. For example,trade off between speed of attaining a new setpoint and system stabilitymay be provided. Achievable objectives are also determined consideringthe capabilities of the PID controler.

[0165] The three PID control gains are determined based on solving anequation of the type illustrated in step 93 using the results of steps90 and 92. The results of step 92 tells the performance metric tooptimize and the system response characteristics of step 90 are used asvalues in the equation illustrated in step 93.

[0166] In the next step the three calculated PID control gains areprovided to the PID controller, as shown in step 94. Based on thesethree numbers the PID controller is expected to provide the closed loopcontrol of the input/output parameters.

[0167] Next, the system is used experimentally to test whether theclosed loop response PID controller works, as shown in step 95. That is,whether it reaches and maintains the set points set by the user, whetherit returns to those setpoints when changes to another set point aremade, and whether it automatically adjusts for aging and hysteresis.According to decision step 96, if it does not meet these objectives,changes are made in the trade-off analysis step 92 and the PID contolgains are correspondingly adjusted. If it works, then closed loopoperation has been enabled for those input-output parameters, as shownin step 97.

[0168] The model values will be used either with an optimal controlalgorithm or in a internal model control technique to determine thecontrol gains, as described in step 93. However, an importantconsideration is to determine the performance objectives for each loop,primarily how fast and with what steady state accuracy, as well as howthe loops act together—the magnitude of cross talk interference, asdescribed in step 91. One important consideration is that the centroidcontrol loop should act the fastest of the loops if sensor data is beingtaken from one region within the plume. Otherwise, the temperature andvelocity measurements will not reflect a consistent sampling point,resulting in at best a transient error, increasing the settling time ofthe system, and possibly driving it unstable. Standard control tuningprocedures can easily incorporate-a specification of desired closed looptime response.

[0169] The desired closed loop settling time performance objective isalso related to how the closed loop system would be used in a productioncell. Since currently, the majority of sensors available on the marketare sensitive to reflected light from surfaces during deposition, aswell as the possibility of observing particles that bounce back into theimaging volume from the surface, it is preferable to not implement thecontroller to act while a part is being coated. Instead, the controlsystem would most effectively be used at the beginning of the partdeposition process, and possibly at intermediate times, but with thetorch brought to a home position where the sensors are located. As such,the time constants of the closed loop system would be chosen to enablethe torch to be brought back to its set point conditions for particleand spray state in a reasonable amount of time, on the order of aminute.

[0170] The complexity that the cross coupling poses is illustrated inthe response of the system to a step change in temperature, whilemeasuring particle velocity and centroid position, as shown in FIGS.16a, 16 b, 16 c. FIG. 16d shows the complex and coordinated response ofthe current I, torch gas flow rate Qt, and carrier gas flow rate Qcgenerated by the controller in response to the change in temperature setpoint command in FIG. 16a that quickly provides temperature at the newdesired set point and restores centroid and velocity to their previousvalues. The advantage of automatically coordinating changes in all threeinputs at the same time is that the time needed to make all the requiredinput changes is minimized and the system makes a robust response tosystem variations and hysteresis. It is not conceivable that a humanoperator could make such a complex, quick adjustment that would maintainor restore all three desired outputs in such a quick fashion.

[0171] The importance of implementing this control system is that itminimizes the variations in plasma spray deposition conditions toachieve consistent set point control, as shown in FIGS. 16a, 16 b, 16 c.As a result, one is able to improve the ability to select the depositionconditions that achieve the desired coating structures, such as crack(or delamination) density and orientation, void density, andconcentration.

[0172] The present inventors have used the control system to investigatethe ability to more tightly engineer crack structure as well as increasethe deposition rate. FIG. 16e shows the results of depositing coatingsat 9 different conditions of temperature and velocity, holding thecentroid position constant where the density of verticle cracks(oriented from 0-45 degress) is plotted. The data shows that the densityof cracks can be varied by a factor of 2.2 by varying set pointtemperature and particle velocity while keeping centroid position fixed.We note that at each point, several test cupons were deposited, and thatfor each substrate, five different points were analyzed. In all cases,the range of variation indicated in this plot is significantly less thenthe standard deviation of the data taken for each point. FIG. 16f plotsthe experimentally determined deposition rate for 9 different depositionconditions of temperature and velocity, where centroid position was heldconstant. The substantial variation in deposition rate (factor of 5) forthese different conditions, indicates the impact of the observed normalvariation of particle temperature and velocity that was shown in FIG. 1ccan have in terms of inducing coating thickness variations. Here,however, these variations are under control. Thus, by maintaining thedesired temperature, velocity, and centroid position to a set point byusing the closed loop control system of the present invention, thesevariations are significantly reduced. In fact they provide the abilityto increase deposition rate and the fraction of particles that getdeposited, and to more closely control deposition thickness.

[0173] The typical variation in temperature and velocity from torchaging and on/off hysteresis (indicated on the plot), would normallyobscure the ability to achieve these different variations. By holdingtemperature and velocity to set points the variability is avoided andthe output parameters can be used to control the coating attributes.

[0174] With regard to deposition rate control, FIG. 16f shows theability to significantly vary the deposition rate under controlledconditions, indicating that the large variations observed in FIG. 1c canbe avoided with the process of the present invention.

[0175] In some coating applications, it is desirable to manufacture agraded coating, werein the composition of the coating varies with thecoating thickness. In practice, however, the 20-50% variation in coatingthickness usually attained without the present invention limited theaccuracy of the structure that could be obtained with such gradedcoatings. However, the closed loop control configurations of thisinvention result in better regulation of deposition rate, and can beused to achieve tighter production of graded coatings. Thus, thedeposition rate control system of FIG. 21a′ can be used in conjuctionwith multiple feed sources (including powders, liquids, solutions ofpowders and liquids, and wires) to coordinate the change in compositionwith a desired coating thickness. For example, the feed rate of a firstmaterial is ramped up over a deposition time period while the feed rateof a second material is ramped down. The closed loop control system isused to maintain overall deposition rate by measuring molten flux rateand spray pattern.

[0176] The same coordination between control structure of depositionrate and feeds for providing a graded composition could also be used inFIGS. 23a, 23 b, and 24. In addition, control structures like those inFIGS. 21a, 21 b, 22 a, 22 b, 25, 26, and 27 can be used for gradedcoatings by using powder feeder controller 110 d to specify the massflux rate of all the materials to be deposited. The changing ratiobetween the different feed materials is set to achieve the desiredgraded composition as a function of deposited coating thickness.

[0177] In the control structure of FIG. 17a, each automatic controller110 a, 110 b has independent control over one input parameter.Alternatively, a decoupled control strategy to minimize the adversecross coupling affects is shown in FIG. 18 in which single MIMOcontroller 110 m coordinates change in all errors from summing junctions116 a, 116 b in one complete control program to control both torchcurrent I and torch gas flow or carrier gas flow. The control program isgenerated from experiments that determine input/output couplingrelationships and then implement those relationships as a computeralgorithm.

[0178] A third input and a third output variable can be provided, asshown in FIGS. 17a, 17 b. Average particle velocity is measured usingsensor 114 c, and this data is fed back to summing junction 116 c whichalso takes in preset velocity 118 c and provides an error signal tocontroller 110 c to control torch gas flow, while carrier gas flow iscontrolled by controller 110 b. Alternatively, single MIMO controller110 m′ coordinates change in all errors from summing junctions 116 a,116 b, 116 c in one complete control program to control torch current I,torch gas flow, and carrier gas flow.

[0179] While these and other output parameters may be measured, thepresent inventors recognized and demonstrated the importance ofcontrolling the spray pattern or its centroid position, and this outputis measured and used for control in all cases. They found three reasonsfor controlling spray pattern or centroid position in real time.

[0180] First, the centroid position reflects the greatest particle flux,which corresponds to the location of the greatest coating thickness.They found enhanced ability to repeatable achieve desired coatingthicknesses by controlling based on centroid position. Preferablycentroid position is kept in the same position relative to torch andsubstrate. This centroid position maybe determined by either a positionweighted integral of the spatial distribution of the light intensityemitted, where intensity, measured by a CCD camera or diode array, is afunction of particle area flux rate and particle temperature, or from anintegral of position weighted molten flux ratio.

[0181] Second, sensors that measure other particle parameters, such asthe individual particle states of particle temperature, velocity, andsize may make their measurement in a very small measurement volume.Under these conditions, it is desirable to have that measurement volumein the region of greatest particle flux density to avoid variation thatintroduced by measuring in different regions of particle distribution58. The present inventors discovered that the location of this greatestparticle flux density changes as the torch ages and/or as torch inputvariables, such as current and gas flow rates vary, so the measurementlocation changes as well—unless the torch is properly controlled asprovided in the present invention.

[0182] Third, the present inventors found that the spray pattern changesbecause of aging of the torch or as other torch inputs are changed tocontrol the particle states. The spray pattern feedback loop thereforecompensates for these variations.

[0183] Forth, by keeping the particle flux at the same position relativeto the plasma jet along the torch centerline, the ability of the torchinputs to change the particle states and deposition rate is improved.

[0184] Controller 110 a, 110 b, 110 c, or 110 m provides ability toobtain the same coating characteristics run-to-run and during the lifetime of the torch electrode, and provides the ability to engineercritical aspects of the coating quality, including coatingmicrostructure, such as crack density and orientation as well asporosity, which determines the coating's performance characteristics.

[0185] The primary particle state control strategy described hereinabove is based on using average measurements of the particle states.However, in reality, it is not simply the average state of the particlethat is important to control and track. Rather, it is more important tocontrol and measure those particles that are incorporated into thecoating. In the cases where mostly the molten particles are incorporatedin the coating and the unmolten particles bounce off, then it isappropriate to develop the basic feedback structure based on statisticsof only those molten particles, as described herein above. In analyzingtypical particle temperature data in FIG. 2, it was found that thevelocity of the molten particles has the same distribution as thevelocity of all the particles. Therefore no separate statistics need bedeveloped for the velocity of molten particles. However, thedistribution of temperature for molten particles was significantlydifferent from the distribution for all particles because of the sharpcut off at the melting temperature. Thus, while the same controlstructure and algorithms are used, the effectiveness of using moltenparticle states for feedback, as shown in FIGS. 19 and 20, significantlyimproves control.

[0186] One method of tuning the control algorithm is to use an adaptivecontroller, as described in section 18.4 of the book, Process Dynamicsand Control, D. E. Seborg, T. F. Edgar, D. A. Mellichamp, Wiley, 1989. Asystem developed by Leeds and Northrup uses step responses tocharacterize the system and uses a pre-determined optimized set of PIDgains based on the identified system characteristics. A similarself-tuning PID system is available from the Foxboro Company based on anexpert system approach.

[0187] It is also possible to develop an estimator, wherein the particlestate spatial variations can be used to help estimate what the realconditions are for the particle states before the spray pattern positioncontrol can be established. The estimator is a numerical algorithm thatrelates the measured state of molten particle flux, which is thencorrelated to other important deposition characteristics such as sprayangle, part temperature, surface shape, in order to estimate the actualdeposition rate on the part surface. For example, the correlation can beexpressed in the form of a least squares fit of these variables to theresulting coating thickness obtained from actual deposition experimentsconducted off line.

[0188] From a production view point it is important that the coatingthickness is maintained within specifications run to run and that thedeposition rate is maximized without adversely impacting coatingquality, that is the ability to consistently meet coatingspecifications. The present inventors recognized that by measuring andcontrolling deposition rate, while at the same time controlling theparticle state and spray pattern, they could better achieve desiredcoating thickness while maximizing deposition rate, and improve coatingquality.

[0189] The present inventors also recognized that control of particleflux rate, including both molten particle flux and unmolten particleflux, can also be important to control coating quality. Molten particleflux affects solidification dynamics as well as deposition rate.Unmolten particle flux affects coating porosity.

[0190] In order to control deposition rate, preferably one measures andcloses the loop around molten particle flux and spatial distribution, asillustrated in FIGS. 21a and 22 a. This molten particle flux rate ismeasured in real time—while the torch is spraying—by measuring theparticle flux rate of those particles above a threshold temperature,such as the melting temperature with molten particle sensor 114 d, asshown in FIG. 21a. [Molten particle flux maybe determined from the dataobtained by the individual particle sensors which provides data on therate of particles measured, as well as the particle state (temperature,velocity, and diameter). Thus, the mass flux rate of molten particlescan be calculated from particles above a threshold temperature (such asthe melting temperature), based on the measured diameter and count rate.For each particle, temperature, velocity, and diameter is measured usingsensors. To obtain the mass flus, the number of particles is counted andthe size of each, from which the mass flux rate is calculated. The sameindividual particle sensors can be used to count molten particle fluxrate by counting only the particles having a temperature above themelting point or another selected temperature representingexperimentally found to be associated with deposition. Molten particleflux reading 112 d is fed to estimator 122 that calculates estimateddeposition rate 112 d′ from parameters including measured moltenparticle flux 112 d, surface angle, spray angle, and surfacetemperature. This estimated deposition rate 112 d′ is subtracted fromdesired and preset deposition rate setpoint 118 d in summing junction116 d. The difference or error in molten particle flux from summingjunction 116 d is provided to powder feeder controller 110 d thatadjusts powder feed rate accordingly.

[0191] Molten particle flux rate is preferably measured for a consistentportion of the spray pattern, such as at the centroid position. Care incontinually measuring at the same consistent position in the spatialdistribution is important. The spatial distribution, and its centroidposition, shifts as the controller adjusts the input conditions ofcurrent, plasma gas flow, and carrier gas flow, and the measurementlocation or locations should follow the spatial distribution.

[0192] At the same time molten particle flux rate is being measured,spatial distribution sensor 114 b detects position of the centroid ofspatial distribution 58. The centroid position reading is subtractedfrom desired and preset centroid position setpoint 118 b in summingjunction 116 b. The difference or error in centroid position fromsumming junction 116 a is fed to particle MIMO controller 120 c whichhas an algorithm to adjust torch gas flow or carrier gas flow to torch40 based on this error signal.

[0193] The present inventors found that varying the powder feed rate wasthe most significant factor in controlling flux rate. Thus, powder feedrate is an appropriate input, as shown in FIGS. 21a and 22 a.Alternatively, standoff distance controller 110 e to vary standoffdistance is provided instead of powder feeder controller 110 d, as shownin FIG. 23a to control molten particle flux since the greater thestandoff distance, the cooler the particles, decreasing the flux rate ofmolten particles reaching surface 46. In this case, standoff distance iscontrolled along with centroid position. Alternatively, standoffdistance can be controlled along with temperature, velocity and centroidposition, as shown in FIG. 23b. MIMO controller 126 receives errorsignals from summing junctions 116 a, 116 b, 116 c to adjust current andboth gas flows to control all three of these output particle parameterswhile standoff distance controller 110 e varies standoff distance tocontrol flux rate of molten particles striking surface 46. In yetanother alternative single MIMO controller 128 can be used to receiveall four error signals and provide signals to adjust all four inputvariables, as shown in FIG. 24.

[0194] The present inventors found that while increasing the powder feedrate increases the flux of molten particles, the deposition efficiencydecreases, as shown in FIGS. 15 and 16. To the extent that the particleflux that is not incorporated into the coating, typically the unmoltenparticles has no impact on the coating quality, then one would only needto focus on the molten particle flux. However, if the flux of unmoltenparticles does affect coating quality, then the flux of unmoltenparticles should also be directly controlled as described herein above.

[0195] Since the ratio of the molten to unmolten particles reflects thedeposition efficiency, which impacts coating economics, controlling theratio can help reduce costs. The total economics are determined by theproduction rate as well as deposition efficiency (which affects per partpowder costs). This suggests that the user could develop a cost equationfor their production operation, that could be minimized to determine theoptimal deposition rate and deposition efficiency, which the closed loopcontrol system would maintain.

[0196] The basic control structure described here can be extended toinclude other factors that are found to affect deposition efficiencysuch as spray angle, surface curvature, and factors such as substratetemperature, which may impact splashing. Thus, in order to controlcoating thickness, the information on the varying surface geometry canbe included in the deposition rate set point in order to achieve uniformcoating thickness for surfaces with varying geometry. Similarly,variations in surface temperature which affect splashing and thusdeposition efficiency are also included in the set point trajectory inorder to achieve the desired coating thickness. Finally, if unmoltenparticle fluxes impact coating structure, then unmolten particle fluxshould be subject to control in a manner similar that described hereinabove for all particles and for molten particles.

[0197] Another way to control the deposition rate is to change thecurrent into the torch, thus changing the fraction of molten particles.Kdep controller 110 f or MIMO controller 124 operate from an errorsignal generated by the difference between deposition rate 112 d′ anddeposition rate set point 118 d is used in FIG. 21a′ and 22 a′ inconjunction with another control loop that measures the molten centroidposition 114 b.

[0198] Alternatively, a sensor, such as an eddy current or laserinterferometer measures coating thickness in real time. Based on themeasurement, torch deposition conditions are adjusted on the fly tocompensate for measured coating thickness variations. One method is todetect and act on thickness variations as the torch rapidly traversesthe part to be coated, such as a turbine blade.

[0199] Preferably, the deposition rate controller is coupled to theparticle state controller to control both coating thickness and coatingquality. Thus, four 4 independent outputs are controlled in real time:deposition rate, particle temperature and velocity, and spray patternposition. There are also 4 inputs to adjust: powder feed rate, current,torch gas flow, and carrier gas flow. Either a decoupled controlarchitecture may be used with 4 independent PID loops, or a centralizedMIMO controller can be used, as shown in FIGS. 21, 22, 23, and 24 toprovide the control. Control of deposition rate and control of particlestate provide ability to optimize deposition rate without adverselyaffecting coating quality since the particle state is held constant.

[0200] The number of unmolten particles in the deposition process canimpact important coating attributes such as porosity. One should alsodirectly control the unmolten particle flux rate relative to the moltenparticle flux. However, since we are also controlling the particletemperature, in particular the temperature of the molten fraction, oneneeds an independent input that can be readily manipulated in real-time.Thus, we add the stand-off distance as fifth input, complementingchanging the particle feed rate. Increasing the standoff distance allowsthe particles to cool more, increasing the flux of unmolten particlesthat strike the substrate surface. Both degrees of freedom are includedto control the absolute value of both fluxes independently.

[0201] Options to take into account the coupling between the inputs,standoff distance, and feedrate and the outputs, molten particle fluxrate and unmolten particle flux rate are shown in FIGS. 25, 26, and 27.A decoupled control structure, feeding back the unmolten flux rate tothe standoff distance in controller 130 and molten flux rate to thepowder feed rate in controller 132 is shown in FIG. 25. This structurecan result in reasonable performance since increasing the feed rateimpacts both flux rates on an absolute basis, while changing thestandoff distance only affects the ratio of the two flux rates.

[0202] Improved performance can be obtained using knowledge of theactual input/output coupling to design MIMO control algorithms toexplicitly account for the coupling, as shown in FIG. 26 with two MIMOcontrollers 126 and 134 and FIG. 27 with single MIMO controller 136 forcontrolling all five parameters.

[0203] An alternate presentation of a generalized control system ispresented in FIG. 28. Torch 40 receives current I from torch powersupply 140, torch gas at a flow rate Qt set by torch gas flow controller142, and powder through injector 48 from powder feeder 144 carried bycarrier gas at a flow rate Qc set by carrier gas flow controller 146.Sprayed particles are sensed by particle state sensor 76 that measurestemperature, velocity, and diameter of particles or characteristics ofthe distributions of those parameters. The sensor data is received andanalyzed in data analysis system 148. Sprayed particles are also sensedby particle distribution sensor 76 c. The various sensor data areprovided as feedback to control algorithm 150 in a computer or in ananalog circuit. Set points 118 are also provided to this controlalgorithm. Torch input parameters are adjusted based on differencebetween the set point and the measured data.

[0204] There are other deposition processes such as twin wire arc sprayand combustion spraying that would benefit from the present invention interms of improving coating quality and production objectives through thecoordinated control of particle temperature, velocity, and spraypattern. The High Velocity Oxygen Fuel (HVOF) thermal spray coatingprocess involves mixing and burning of oxygen and fuel in a combustionchamber. The hot combustion gases are accelerated to high, oftensupersonic, velocities through a converging/diverging nozzle. Thematerial to be sprayed is injected into the hot gas steam. The materialis accelerated and heated by the combustion gases and is deposited onthe substrate part to be coated. A similar principle is used in lowervelocity combustion thermal spray process, except the hot combustiongasses are released through a converging nozzle at lower velocities thenHVOF. In a twin-wire arc spray process, an electrical arc discharge isestablished between a pair of electrodes, where at least one electrodeis a wire composed of the material to be sprayed. The arc melts the wiremade of the material to be sprayed, and a flow of gas through the arcbreaks the melted material into droplets while propelling those dropletsin the direction of the substrate to be coated. Wire is fed into the arcto maintain a constant arc gap as the end of the wire melts.

[0205] With HOVF and low velocity combustion, it has been found that theparticle temperature can effectively be changed by varying the fuel tooxygen mixture ratio. This can be done by individually adjusting thefuel and oxygen flow rates. The fuel to oxygen mixture ratio changesaffects the temperature of the combustion gasses, thereby affecting thetemperature of the particles. By measuring the particle temperature, thefuel and oxygen flow rates can be adjusted so that the particle flux ismaintained at the desired temperature. The particle velocity has beenfound to be a function of combustion chamber pressure. The chamberpressure can be manipulated by changing the total mass flow rate ofoxygen and fuel at a given mixture ratio. Thus, one can adjust the totalmass flow rate in response to a direct measurement of the chamberpressure, or additionally, in terms of a feedback signal form ameasurement of particle velocity.

[0206] The spray pattern of the high or low velocity oxygen fuel sprayequipment is monitored by a video camera. Coating particles are injectedinto the hot combustion gas flow by suspending the coating particles ina carrier gas, and directing the flow of carrier gas into the hotcombustion gas flow which is similar to the plasma arc spray process.The spray pattern of particles can be controlled by altering the rate offlow of the carrier gas by means of carrier gas controller. Similarly,the control of the mass flux rate can be achieved by varying the powdermass flow rate into the carrier gas by adjusting the particle feeder.

[0207] In the twin wire arc process, a power supply provides energy toan arc in a torch. The arc in the torch is established between a pair ofwires. A gas flow is established through the arc that will acceleratemelted electrode wire material in the form of a spray. Sensors describedfor the plasma spray process can be used to measure the velocity of theparticles as well as their temperature. Thus, the particle temperaturecan be controlled by adjusting the power to the arc from the powersupply in response to variations of particles temperature, whileparticle velocity can be controlled by adjusting the flow rate of thegas flow through the arc by altering the setting of an arc gas flowcontroller. The mass flux of the resulting droplets can be measured bythe individual particle sensors, and used to determine both the totalmass flux rate or the molten flux rate, and used to adjust the wire feedrate, power, and gas flow rates. In addition, the distribution of thewire arc spray pattern, such as its width can be controlled by adjustingthe gas flow, power, and wire feed rates. The required control structure(ie whether a decoupled or coupled control structure would work, therequired inputs, and input/output pairs) would follow the proceduredescribed for the plasma spray control, FIG. 7.c and 7.d. Thus, firstone would determine the input/output gains and the related wire arcsystem (or HVOF) transfer matrix, scale the matrix, and evaluate thecondition number to determine the feasibility of controlling all threedegrees of freedom. Next, one would evaluate the RGA array to determinethe level of interaction between various loops, and whether a decoupledcontrol structure is practicable. Lastly, one would then determine thespecific input output pairs.

[0208] Other methods for adjusting the velocity, temperature, and spraypattern of thermal spray equipment may be used. For example, it may bepossible to control the temperature of a fuel-oxygen torch type sprayequipment by blending a controlled proportion of an inert gas with theoxygen. The temperature of the particles would be a function of the massflow of oxygen, the mass flow of fuel, and the mass flow of inert gas.Similarly, because the velocity of particles is a function of thevelocity of gas flow out of the combustion chamber or through the arc,and the velocity of gas flow is a function of nozzle shape, particlevelocity could be controlled through adjustment of an adjustable venturenozzle.

[0209] While several embodiments of the invention, together withmodifications thereof, have been described in detail herein andillustrated in the accompanying drawings, it will be evident thatvarious further modifications are possible without departing from thescope of the invention. Nothing in the above specification is intendedto limit the invention more narrowly than the appended claims. Theexamples given are intended only to be illustrative rather thanexclusive.

What is claimed is:
 1. A method of depositing a material on a substrate,the method comprising the steps of: a) providing a plasma spray torchhaving electrodes; b) providing a first gas into said plasma spraytorch, said first gas having a first gas flow rate; c) providing acontrollable power supply for providing a current across said electrodesfor generating a plasma in said first flow of gas; d) providing a powdermaterial; e) providing a second gas for carrying said powder materialand directing said second gas carrying said powder material into saidplasma; f) heating said powder material in said plasma and acceleratingparticles of said powder material from said spray torch with said firstgas; g) measuring a temperature of said particles; h) measuring aspatial distribution of said particles or measuring a parametercharacteristic of said spatial distribution of said particles; and i)adjusting current from said controllable power supply and adjusting saidfirst gas flow rate or said second gas flow rate to obtain a presettemperature of said particles and a preset spatial distribution or apreset parameter characteristic of said spatial distribution of saidparticles.
 2. A method of spraying a material as recited in claim 1,wherein said parameter characteristic of said spatial distributioncomprises centroid position.
 3. A method of spraying a material asrecited in claim 1, wherein said parameter characteristic of saidspatial distribution comprises a peak of said distribution or a width ofsaid distribution.
 4. A method of spraying a material as recited inclaim 1, further comprising the step of measuring velocity of particlesat a specified distance from the torch, adjusting said controllablepower supply to adjust said current, and adjusting said first gas flowrate or said second gas flow rate to obtain a preset velocity,temperature, and spatial distribution or a parameter characteristic ofsaid spatial distribution.
 5. A method of spraying a material as recitedin claim 1, wherein in said adjusting step (i), said adjusting is basedon a second spatial distribution of particles having a temperature abovea specified value or on a second parameter characteristic of a spatialdistribution for particles having a temperature above a specified value.6. A method of spraying a material as recited in claim 5, wherein saidspecified value of temperature is about equal to melting temperature ofsaid material.
 7. A method of spraying a material as recited in claim 5,further comprising the step of measuring mass flux of said particleshaving a temperature above a specified value at a specified distancefrom said spray torch.
 8. A method of spraying a material as recited inclaim 7, further comprising the step of adjusting feed rate of saidpowder, adjusting standoff distance, or adjusting current to obtain adesired measurement of said mass flux of particles above said meltingtemperature at said specified distance from said spray torch.
 9. Amethod of spraying a material as recited in claim 8, further comprisingthe step of measuring velocity of sprayed particles at a specifieddistance from the torch and adjusting current or flow rate of said firstgas or flow rate of said second gas to obtain a preset velocity.
 10. Amethod of spraying a material as recited in claim 1, further comprisingthe step of measuring mass flux of particles having a temperature belowa specified value at a specified distance from said spray torch.
 11. Amethod of spraying a material as recited in claim 10, further comprisingthe step of adjusting feed rate of said powder or standoff distance toobtain a preset measurement of mass flux of particles having atemperature below said specified value at said specified distance fromsaid spray torch.
 12. A method of spraying a material as recited inclaim 11, further comprising the step of obtaining a preset velocity,temperature, spatial distribution or parameter characteristic of spatialdistribution, and mass flux of particles having said temperature belowsaid specified value.
 13. A method of spraying a material as recited inclaim 1, further comprising the steps of experimentally measuring inputvs. output relations, choosing to control those input parameters thatprovide substantial gain to adjust each output parameter; and choose apairing of output sensor data to input parameter based on that choice.14. A method of spraying a material as recited in claim 1, furthercomprising the step of providing feedback of sensor data to a controllerof input variables, wherein interaction between feedback from multiplesensors does not create system instability.
 15. A system, comprising asensor, an automatic controller, an actuator, and an input variable,said sensor for measuring a spatial distribution of particles or fordetecting a spatial parameter characteristic of said spatialdistribution of particles, said input variable being one that effectssaid spatial distribution of particles, said automatic controller forreceiving said spatial distribution or said spatial parameter data fromsaid sensor and directing said actuator, said actuator for adjustingsaid input variable as directed by said automatic controller based onsaid spatial data.
 16. A system as recited in claim 15, wherein saidsystem comprises a deposition system.
 17. A system as recited in claim16, wherein said deposition system comprises a spray deposition system.18. A system as recited in claim 15, wherein said sensor is formeasuring a spatial distribution of moving particles or for detecting aspatial parameter characteristic of said spatial distribution of saidmoving particles.
 19. A system as recited in claim 15, wherein saidparticles comprise powder particles.
 20. A system as recited in claim15, wherein said input variable comprises current, a gas flow rate, apowder feed rate, a wire feed rate, a liquid feed rate, or a suspensionfeed rate.
 21. A system as recited in claim 15, wherein said automaticcontroller comprises a processor, control logic, and filteringalgorithms.
 22. A system as recited in claim 15, wherein said sensor isset to obtain sensor data only from particles having a presetcharacteristic or said automatic controller is set to analyse sensordata only for particles having said preset characteristic.
 23. A systemas recited in claim 15, wherein said preset characteristic comprisestemperature.
 24. A system as recited in claim 15, wherein saidtemperature is about equal to melting point of said particles.
 25. Asystem as recited in claim 15, further comprising a second sensor, asecond actuator, and a second input variable, said second sensor formeasuring temperature of particles, said automatic controller forreceiving said temperature measurement from said sensor and directingsaid actuator, said actuator for adjusting said input variable asdirected by said automatic controller based on said temperaturemeasurement.
 26. A system as recited in claim 25, further comprising athird sensor, a third actuator, and a third input variable, said thirdsensor for measuring velocity of particles, said automatic controllerfor receiving said velocity measurement from said sensor and directingsaid actuator, said actuator for adjusting said input variable asdirected by said automatic controller based on said velocitymeasurement.
 27. A system as recited in claim 15, further comprising thestep of measuring mass flux of particles having a temperature below aspecified value at a specified distance from said spray torch.
 28. Asystem as recited in claim 15, further comprising the step of adjustingfeed rate or standoff distance to obtain a preset measurement of massflux of particles having a temperature below said specified value atsaid specified distance from said spray torch.
 29. A system as recitedin claim 28, further comprising the step of obtaining a preset velocity,temperature, spatial distribution or parameter characteristic of spatialdistribution, and mass flux of particles having said temperature belowsaid specified value.
 30. A system as recited in claim 15, furthercomprising the steps of experimentally measuring input vs. outputrelations, choosing to control those input parameters that providesubstantial gain to adjust each output parameter; and choose a pairingof output sensor data to input parameter based on that choice.
 31. Asystem as recited in claim 15, further comprising the step of providingfeedback of sensor data to a controller of input variables, whereininteraction between feedback from multiple sensors does not createsystem instability.
 32. A system for depositing a material on asubstrate, comprising: a spray torch having electrodes; a first gas forinjecting into said spray torch, said first gas having a first gas flowrate; a controllable power supply for providing a current across saidelectrodes; a controllable device for feeding a material into a regionadjacent said electrodes, wherein said material is heated in said regionand particles of said material are accelerated from said spray torchwith said first gas; a first sensor for measuring a temperature of saidsprayed particles; a second sensor for measuring a spatial distributionof said sprayed particles or measuring a parameter characteristic ofsaid spatial distribution of said sprayed particles; a current actuatorfor adjusting current from said controllable power supply; a firstactuator for adjusting said first gas flow rate; a second actuator foradjusting said controllable device for feeding said material; and acontroller to receive data from said first sensor and said second sensorand to direct operation of said first actuator and of said secondactuator to obtain a preset temperature and a preset spatialdistribution or a preset parameter characteristic of said spatialdistribution of said sprayed particles.
 33. A system as recited in claim32, wherein said controllable device for injecting a material includes aholder for holding particles of a material, wherein said controllabledevice further includes a second gas for carrying said particles intosaid region.
 34. A system as recited in claim 33, wherein said secondactuator is for controlling flow of particles of said material into saidsecond gas, wherein the system further comprises a third actuator foradjusting flow of said second gas.
 35. A system as recited in claim 34,wherein said first gas is ionized in said region adjacent saidelectrodes to form a plasma and wherein said particles of said materialare heated in said plasma.
 36. A system, comprising a sensor and acontroller for controlling an actuator, said sensor for detecting aspatial distribution of particles, wherein said spatial distribution ofparticles has a densest portion, wherein said sensor is set to receivedata from a plurality of positions within said spatial distribution andto automatically locate and receive sensor data from said densestportion, wherein said controller is set to control said actuator basedsolely on said data from said densest portion.
 37. A system as recitedin claim 36, wherein said densest portion comprises a centroid of saidspatial distribution.
 38. A system as recited in claim 36, wherein saiddensest portion comprises a peak of said spatial distribution.
 39. Asystem as recited in claim 36, wherein said sensor comprises a camerafor detecting location of highest intensity optical emission.
 40. Amethod of spraying a coating on a substrate comprising the steps of: a)spraying a material with a spray tool to provide a spatial distributionof sprayed particles; b) measuring said spatial distribution of sprayedparticles or measuring a parameter of said spatial distribution ofsprayed particles; and c) providing automatic closed loop control oversaid spatial distribution of sprayed particles.
 41. A method as recitedin claim 40, wherein said spray tool comprises a plasma spray tool. 42.A method as recited in claim 40, wherein said parameter comprisescentroid position.
 43. A method as recited in claim 40, wherein saidparameter comprises peak of said spatial distribution position or widthof said spatial distribution.
 44. A method as recited in claim 40,further comprising the step of measuring temperature or velocity ofsprayed particles and providing automatic closed loop control over saidtemperature or velocity of sprayed particles.
 45. A method as recited inclaim 40, further comprising the step of measuring temperature andvelocity of sprayed particles and providing automatic closed loopcontrol over said temperature and said velocity of sprayed particles.46. A method as recited in claim 40, wherein said spray tool has aplurality of adjustable input parameters, wherein said closed loopcontrol comprises automatically adjusting said input parameters toachieve a preset spatial distribution or a preset parameter of saidspatial distribution.
 47. A method as recited in claim 40, furthercomprising the step of spraying a production part, wherein saidspraying, measuring, and providing closed loop control steps areprovided at a separate location from the step of spraying the productionpart.
 48. A method of spraying a material as recited in claim 1, furthercomprising the step of providing a second powder material and adjustingfeedrates of said power material and said second powder material toprovide a graded composition.
 49. A method of spraying a coating on asubstrate comprising the steps of: a) spraying a material with a spraytool; b) measuring an output parameter of said spray tool; c) providingautomatic closed loop control over said output parameter; and d) settingset points of said output paramter to achieve a desired coatingporosity.
 50. A method of spraying a material as recited in claim 1,wherein said output paramter comprises temperature, velocity, or spatialdistribution of sprayed particles.
 51. A method of spraying a materialas recited in claim 1, wherein step (b) comprises measuring a pluralityof said output parameters and said step (c) comprises providingautomatic closed loop control over each of said plurality of outputparameters and said step (c) comprises setting set points of each ofsaid output paramter to achieve a desired coating porosity.